1. Field
The present invention generally relates to material blenders, and more specifically to automated loading of a desired amount of each material into a material blender.
2. Prior Art
Mixers and blenders are used in the plastics industry for processing chemicals in a normal state of pelletization. This may include natural material, regrind material, and colorant or other additives. Regrind material may be natural material which has been put through a granulator (also known as a grinder) after being processed into a part. For example, the part may have been rejected by quality control and can be turned into regrind material and re-processed if it is thermoplastic material. The colorant and additives in pellet form can be added to the natural and/or regrind material to change the chemical characteristics of the natural and/or regrind material. Colorant gives the part its color. An example of an additive may be a UV stabilizer added to the material to provide ultraviolet protection to the part so it does not fade or degrade in the sun.
It is normal in most manufacturing plants for processing machines to use natural material and to add a percentage of regrind material to it, typically 10 to 30 percent. Colorant added to the natural material may typically be about 1 to 10 percent and additives may typically be about 1 to 10 percent of the total resulting heterogeneous mixture. Accuracy for the recipe of mixing these materials is important, as these materials can be very expensive. Furthermore, if the parts made from these materials do not meet specifications, the parts are rejected, resulting in lost productivity and profitability.
Most manufactures use one of the following methods for adding regrind, colorant, and additive materials to natural material or feed stock: volumetric blending or weight-based blending. Volumetric blending determines the volume of each ingredient required based on percentages of a total volume to be produced. The equipment for this method is not very expensive, but it is also not very accurate. Often more colorant and additives must be added to make good parts than are called for in the material recipe. Accuracy for the volumetric blending method may be limited to plus or minus several full percentage points, due to variations in shape, size, density, and/or weight of the different materials being blended.
The weight-based blending method uses weigh scales or gravimetric blenders, with a greater resulting accuracy than the volumetric blending method. For example, the accuracy of the weight-based blending method may be plus or minus approximately 0.1% for additives and plus or minus approximately 1% for regrind. The gravimetric mixers often use load cells which have to be calibrated with bins set on top of them for holding each of the materials. The load cell gravimetric mixer is complex, large, and expensive, including bins, doors to the top of bins, doors at the bottom of bins, actuators to open doors of the bins, augers to feed materials to the bins, rotating blades on a shaft under the bins to mix materials, etc. Furthermore, if the mixed materials are conveyed directly from the gravimetric mixers to subsequent processing machines, the material may segregate due to different weight, density, and size, resulting in inconsistent parts that do not meet specifications. The vibration of the manufacturing floor can also cause load cells to come out of calibration, resulting in a loss of accuracy and the need for time-consuming re-calibration. Loading the gravimetric blender can also be a time-consuming, labor-intensive process. Automatic loaders are either purchased separately for loading materials into existing gravimetric loaders, or an operator loads the materials manually.
Therefore, there is a need for an improved method of material blending which does not suffer from the limitations of the prior art.